1. Field of the Invention
This invention relates to a power conversion system, and more particularly to a power conversion system which converts an AC power to a DC power by a converter and further converts it to an AC power of an arbitrary frequency by an inverter to drive and control an induction motor and can thereby suppress the torque ripple caused by the pulsation of the DC link voltage.
2. Description of the Related Art
In general, in a system for driving and controlling an induction motor by converting an AC power from a power source to a DC power by a converter, smoothing this DC power by a DC capacitor and further converting the smoothed DC power to an AC power of an arbitrary frequency, DC link voltage pulses because of higher harmonics superposed on the current flowing to the DC capacitor from the converter.
In case of three phase power source, a pulsating frequency of DC link voltage is 6-times frequency 6f.sub.so of a supply frequency f.sub.so, and in case of a single phase power source, it is two times frequency 2f.sub.so of supply frequency f.sub.so.
On the other hand, when producing three-phase AC power from the DC voltage by an inverter, a beat phenomenon of phase current and torque ripple are generated by fluctuation of the DC link voltage, and they become problems.
This beat phenomenon of the phase current is a phenomenon wherein phase current is oscillated by a differential frequency between a fluctuating frequency of the DC link voltage and an inverter output frequency f.sub.i, and regarding the torque, the torque ripple of the fluctuating frequency of the DC link voltage is generated.
In particular, in case of a single phase power source this will become a problem because a fluctuating frequency of the DC link voltage is low.
When a supply frequency is 50/60 Hz, a fluctuating frequency of the DC link voltage will become 100/120 Hz.
So, in recent years, regarding the beat phenomenon of the phase current, its cause is assumed to be attributable to an unbalance between plus and minus voltages superposed on the phase voltage, and a control method to remove this unbalance is proposed in, for instance, Literature 1 ("Method to Suppress Beat Phenomenon in Converter/Inverter System"), Section D of Collected Paper of The Institute of Electrical Engineers of Japan, Vol. 109, No. 5, P. 363).
Hereinafter, a conventional method to suppress a phase. current beat phenomenon will be described using FIG. 37.
FIG. 37 is a block diagram illustrating an example of a schematic construction of a conventional power conversion system.
In FIG. 37, this system is in such a construction that a single phase AC power from a single phase power source 1 is converted to a DC power by a single-phase converter 2, is smoothed by a DC capacitor 3 and further, is converted to an AC power of an arbitrary frequency by an inverter 4 so as to drive and control an induction motor 6.
On the other hand, a slip angular frequency reference .omega..sub.s * is calculated by a slip frequency controller 29, is added to a velocity (angular frequency) .omega..sub.r of induction motor 6 detected by a velocity detector 7, and thus an output angular frequency reference .omega..sub.i * of inverter 4 is calculated.
Further, from a DC link voltage V.sub.dc, that is a terminal voltage of DC capacitor 3, detected by a voltage detector 8, a mean value V.sub.dc * of DC link voltage .sub.dc is calculated by an mean value computing unit 30 and a fluctuation quantity .DELTA.V.sub.dc from this mean value V.sub.dc * is computed by a fluctuation quantity computing unit 18. Then, a value obtained by dividing this fluctuation quantity .DELTA.V.sub.dc by this mean value V.sub.dc * is input to a gain/phase compensator 20.
Further, a gain and phase compensated sine wave is output from gain/phase compensator 20 by compensating the gain and the phase to a sine wave of two times of frequency 2f.sub.so of supply frequency f.sub.so.
This output is a compensating value .DELTA..omega..sub.i of an output angular frequency of inverter 4, and is added to output angular frequency reference .omega..sub.i * of inverter 4 to obtain an output angular frequency .omega..sub.i of inverter 4.
Then, this output angular frequency .omega..sub.i of inverter 4 is integrated by an integrator 11 to obtain an output voltage phase angle .theta..sub.i of inverter 4, which is input to a gate controller 17.
In this gate controller 17, a gate signal is generated for inverter 4 based on this output voltage phase angle .theta..sub.i of inverter 4.
By the way, in the above-described Literature 1, a method for setting a gain/phase compensator is analytically obtained in connection with the current beat.
Further, it is pointed out that the torque ripple can be suppressed by finely adjusting gain and phase.
However, regarding the point that the torque ripple can be suppressed by finely adjusting gain and phase, only its effect is pointed out from the result of simulation, and nothing is stated as to its basis. Further, although it is pointed out that the adjustment is required corresponding to the operating state, it is not theoretically backed. Therefore, an adjusting method is not clear and there is such a problem that the actual adjustment is difficult.